Blood Culture

Test Overview

A blood culture is a test to find an
infection in the blood. The blood does not normally have any bacteria or
fungi in it. A blood culture can show what bacteria or
fungi are in the blood.

A bacterial infection in the blood, called
bacteremia, can be serious because the blood can spread the bacteria to any
part of the body. A blood infection most often occurs with other serious
infections, such as those affecting the lungs,
kidneys, bowel,
gallbladder, or heart valves.

A blood
infection may also develop when the
immune system is weak. This can occur in infants and
older adults, and from disease (such as cancer or
AIDS) or from medicines (such as
corticosteroids or
chemotherapy) that change how well your body can fight
infections (immunity).

To test for an infection in the blood, a
sample of blood is collected and placed in a cup with special substances that
allow the bacteria or fungus to grow. The type of bacteria or fungus that grows
is checked with chemical tests and by looking at the culture under a
microscope. Two or three blood samples from different veins are often taken to
make sure a bacteria or fungus is not missed. If no bacteria or fungus grows,
the blood culture is called negative. A blood culture is often done when a
person has a fever because this is the time when the bacteria or fungus is most
likely to have spread to the blood.

Find the cause of an
unexplained fever or
shock or a person becoming extremely ill.

How To Prepare

You do not need to do anything before
having this test. Tell your health professional if you have recently taken
antibiotics.

How It Is Done

The health professional drawing blood
will:

Wrap an elastic band around your upper arm to
stop the flow of blood. This makes the veins below the band larger so it is
easier to put a needle into the vein.

Clean the needle site
carefully with alcohol or iodine so skin bacteria will not get in the blood
sample.

Put the needle into the vein. More than one needle stick
may be needed.

Attach a tube to the needle to fill it with
blood.

Remove the band from your arm when enough blood is
collected.

Put a gauze pad or cotton ball over the needle site as
the needle is removed.

Put pressure to the site and then a
bandage.

Blood is often collected from two or three different body
sites. Or it may be collected at two different times a few hours apart.

Some people may have long-term catheters placed in a major vein because
they are receiving chemotherapy or nutrition supplements for weeks or months at
a time. For these people, blood for a blood culture will be collected from
their catheters for this test.

How It Feels

The blood sample is taken from a vein in
your arm. An elastic band is wrapped around your upper arm. It may feel tight.
You may feel nothing at all from the needle, or you may feel a quick sting or
pinch.

Risks

There is very little chance of a problem from
having a blood sample taken from a vein.

You may get a small bruise at the site. You
can lower the chance of bruising by keeping pressure on the site for several
minutes.

In rare cases, the vein may become swollen after the
blood sample is taken. This problem is called phlebitis. A warm compress can be
used several times a day to treat this.

Ongoing bleeding can be a
problem for people with bleeding disorders. Aspirin, warfarin (such as Coumadin), and
other blood-thinning medicines can make bleeding more likely. If you have
bleeding or clotting problems, or if you take blood-thinning medicine, tell
your doctor before your blood sample is taken. Firm pressure may be needed on
the puncture site.

Results

A blood culture is a test to find an
infection in the blood. Most bacteria can be seen in the culture in 2 to 3
days, but some types can take 10 days or longer to show up. Fungus can take up
to 30 days to show up in the culture.

Blood culture

Normal:

No bacteria or fungus is
found. Normal culture results are called negative.

Abnormal:

Bacteria or fungus grows in
the culture. Abnormal culture results are called positive.

If bacteria are found in the culture, another test is
often done to find the best
antibiotic that will kill the bacteria. This is called
sensitivity or susceptibility testing. Sensitivity
testing is important so the blood infection is treated correctly. This also
helps prevent bacteria from becoming
resistant to antibiotics.

What Affects the Test

Reasons you may not be able to
have the test or why the results may not be helpful include:

If you have taken antibiotics recently. These
medicines may stop the growth of bacteria in the culture.

If the blood sample is contaminated by bacteria or fungus on the
skin.

If the blood test misses the time when bacteria actually are in
the blood. Blood culture tests are done at several different times to make
sure bacteria are not missed.

If the blood test is not done
correctly or the blood sample is not processed properly. In these cases, a
false-positive or
false-negative result could occur.

What To Think About

Some types of bacteria infect the blood when
another infection of the kidneys, throat, lungs, or another part of the body is
present. This may not mean a serious infection of the blood.

About
5% of blood cultures are contaminated with normal skin bacteria (a type of
staph bacteria). So it is sometimes hard to see whether the bacteria
that grow in the culture are the cause of the blood infection or not. This is
why more than one blood sample is taken. When the same bacteria grow in several
blood cultures, it is likely that those bacteria are in the blood and are
causing the infection. When staph bacteria grow in the culture in less than 48
hours, it is likely that the staph bacteria are in the blood and are causing
the infection.

A culture that does not grow any bacteria does not
always mean a blood infection is not present. The amount of blood taken, the
timing of the blood sample, the type of culture done, and recent use of
antibiotics can affect the growth of bacteria in the culture.

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